[Laser] New Scientist

kirk [email protected]
Wed, 21 May 2003 20:17:25 -0600


 http://www.newscientist.com/news/news.jsp?id=ns99993750

      Alchemy with light shocks physicists


      19:00 21 May 03

      Exclusive from New Scientist Print Edition

      Claims of "unexpected and stunning new physical phenomena" are rare in
the abstract of a reputable scientific paper. But the latest report by
photonics crystal pioneer John Joannopoulos and his group at MIT, soon to be
published in Physical Review Letters, does not disappoint.

      The researchers document the ultimate control over light: a way to
shift the frequency of light beams to any desired colour, with near 100 per
cent efficiency. "The degree of control over light really is quite
shocking," comments photonics expert Eli Yablonovitch at the University of
California, Los Angeles.

      If the effect can be harnessed, it will revolutionise a range of
fields � turning heat into light, for example, or prized terahertz rays.
Right now, the only way to shift the frequency of a light beam involves
sending an extremely intense light pulse � with a power of many megawatts or
even gigawatts � along next to it.

      This interacts with the first beam and alters its frequency, but the
technique is expensive, requires high-power equipment, and is generally
pretty inefficient. But when Joannopoulos and his colleagues Evan Reed and
Marin Soljacic investigated what happens when shock waves pass through a
device called a photonic crystal, they discovered a completely unexpected
effect.


      Hall of mirrors


      Photonic crystals, which are made by sandwiching together layers of
material that bend light in different ways, can be designed to reflect some
frequencies while letting others through. They are used to steer light
through circuits in the same way that electronic circuits direct electric
current.

      From computer simulations, the team found that shock waves passing
through a crystal alter its properties as they compress it. For example, a
crystal that normally allows red light through but reflects green light
might become transparent to green light and reflect red light instead.

      The researchers worked out that if a photonic crystal is designed in a
certain way, incoming light can get trapped at the shock wave boundary,
bouncing back and forth between the compressed part of the crystal and the
uncompressed part, in a "hall of mirrors" effect.

      Because the shock wave is moving through the crystal, the light gets
Doppler shifted each time it bounces off it. If the shock wave is travelling
in the opposite direction to the light, the light�s frequency will get
higher with each bounce, while if it travelling in the same direction, the
frequency drops.

      After 10,000 or so reflections, taking a total of around 0.1
nanoseconds, the light can shift dramatically in frequency � from red up to
blue, for example, or from visible light down to infrared. By changing the
way the crystal is built up, it is possible to control exactly which
frequencies can go into the crystal and which come out. "We ought to be able
to do things that have never been possible before," Joannopoulos told New
Scientist.


      Shooting bullets


      The technique can even focus a wide range of frequencies into a narrow
band, something no other known method can do, says Joannopoulos. Normal
colour filters merely let through the desired frequencies and chop the
others away, so much of the energy is lost.




            Related Stories


            Desktop kit slows light to a crawl
            2 April 2003

            Light turns into glowing liquid
            7 July 2002

            Speed of light broken with basic lab kit
            16 September 2002


            For more related stories
            search the print edition Archive



            Weblinks


            Photonic Crystal Research, MIT

            Opto-Electronics Group, University of California

            Physical Review Letters

            Sailor Research Group, University of California

            Lawrence Livermore National Laboratory



      The team is now collaborating with researchers at Lawrence Livermore
National Laboratory to demonstrate the effect. Initially they will generate
shock waves by shooting bullets at photonic crystals. This would destroy the
crystal, but not before the light has had time to shift. Eventually, sound
waves should do the job just as well, they say. "It�s really practical, and
potentially even easier to do than with actual shock waves," says Reed.

      The work is impressive, says materials chemist Michael Sailor at the
University of California, San Diego, whose team has developed flexible,
biodegradable photonic crystals. He says he now plans to test the phenomenon
for himself.

      Besides making devices such as light bulbs and solar cells more
efficient, the method would also help to keep optical telecommunications
networks moving. At the moment, many light frequencies are bounced down
optical fibres simultaneously. If a particular frequency is being used to
capacity, then optical switches could shift light beams to a frequency where
there is still capacity to spare.

      Another benefit of pushing the frequency of light downwards would be
the ability to make terahertz radiation. Terahertz rays, in the range
between microwaves and infrared, hold great promise for medical imaging, as
they are easier to focus and less damaging than X-rays (New Scientist print
edition, 14 September 2002, p 34). But they are not yet widely used as they
have been too difficult to produce.


      Charles Choi





--- StripMime Report -- processed MIME parts ---
multipart/mixed
  multipart/alternative
    text/plain (text body -- kept)
    text/html
  application/octet-stream
The reason this message is shown is because the post was in HTML
or had an attachment.  Attachments are not allowed.  To learn how
to post in Plain-Text go to: http://www.expita.com/nomime.html  ---